1. Technical Field
The present invention relates to a pressure sensor using a pressure-sensitive element and a diaphragm and, specifically, to a technique to reduce an error of a pressure measurement value of the pressure sensor due to a temperature change caused by the fact that different types of materials are combined.
2. Description of the Related Art
In the related art, a pressure sensor using a piezoelectric resonator as a pressure-sensitive element is known as a water pressure meter, a barometer, a differential pressure gauge or the like. The piezoelectric resonator includes an electrode pattern formed on a plate-shaped piezoelectric substrate having a detection axis set in the direction of detection of a force, and being configured to vary in resonance frequency of the piezoelectric resonator when a pressure is applied thereto in the direction of the detection axis, whereby the pressure is detected from the variation in the resonance frequency. In JP-A-2007-57395, a pressure sensor according to a first related art is disclosed. FIG. 18 shows a pressure sensor according to the related art. A pressure sensor 201 in the related art includes an air-tight case 202 having a vacuum or inactive atmosphere in the interior thereof, a first pressure input port 203a and a second pressure input port 204a formed respectively on opposing first and second wall surfaces 203 and 204 of the air-tight case 202 so as to penetrate therethrough, a cylindrical first bellows 210 being fixed at an opening at one end thereof to the first wall surface 203 and having an axial hole communicating with the first pressure input port 203a, a cylindrical second bellows 211 being fixed at an opening at one end thereof to the second wall surface 204, having an axial hole communicating with the second pressure input port 204a arranged in series with the first bellows 210, a resonator bonding base 215 fixedly arranged between other ends 210a and 211a of the first and second bellows 210 and 211, a thin-plate shaped piezoelectric resonator 220 supported by the resonator bonding base 215, a piezoelectric reinforcing panel 221 arranged at a position opposing the piezoelectric resonator 220 with the second bellows 211 interposed therebetween, and an electronic oscillator 230 in conduction with the electrode pattern on the piezoelectric resonator.
The piezoelectric resonator 220 is fixed at one end thereof to the second wall surface 204, and at the other end thereof to the resonator bonding base 215. The piezoelectric reinforcing panel 221 is fixed at both end portions by the second wall surface 204 and the resonator bonding base 215. The resonator bonding base 215 and an inner wall of the air-tight case 202 are fixed by a spring for a reinforcing panel to enhance the durability for an impact in the X-axis direction.
The piezoelectric resonator 220 includes a configuration formed with an electrode on a quartz crystal substrate, for example. The resonator bonding base 215 includes a base portion 215a configured to be fixed in a state of being interposed between the other ends 210a and 211a of the both bellows 210 and 211, and a supporting strip 215b projecting from an outer periphery of the base portion 215a toward the second wall surface, and the other end portions of the piezoelectric resonator 220 and the piezoelectric reinforcing panel 221 are both connected to the supporting strip 215b. 
The pressure input ports 203a and 204a communicate with axial holes in the interiors of the bellows 210 and 211 respectively, while the axial holes in the interiors of the bellows are maintained in a non-communication state by the base portion 215a of the resonator bonding base 215. Therefore, the position of the resonator bonding base 215 is moved forward and backward in the direction of the axes of the bellows in association with the expansion and contraction of the bellows due to the pressure difference of between pressures P1 and P2 supplied from the both pressure input ports 203a and 204a. The piezoelectric resonator 220 fixed at one end thereof to the resonator bonding base 215 and at the other end thereof to the second wall surface 204 is deformed by receiving a mechanical stress in the axial direction by a pressure transmitted from the resonator bonding base 215, so that the specific resonance frequency varies. In other words, the piezoelectric substrate is excited by a distribution power to an excitation electrode in a state in which the electronic oscillator 230 arranged at adequate positions in the air-tight case 202 in an air-tight state is connected to the excitation electrode on the piezoelectric substrate constituting the piezoelectric resonator 220, and the pressure P1 or P2 is calculated by the output frequency at this time.
According to the pressure sensor 201 in the related art, when the pressure P1 is input to the first pressure input port 203a, a force corresponding to the pressure is applied to the piezoelectric resonator 220 and the piezoelectric reinforcing panel 221. Because of the presence of the piezoelectric reinforcing panel 221, only a force in the direction of a longitudinal side (Y-axis direction in the drawing in the case of a quartz crystal resonator) is applied to the piezoelectric resonator 220, so that primary pressure-frequency characteristics of the piezoelectric resonator demonstrates a quadratic curve. Therefore, the resonance frequency of the piezoelectric resonator 220 changes linearly according to the pressure P1 so that the pressure sensor 201 having high degree of accuracy is obtained.
However, in the related art, it is difficult to bring the linear expansion coefficient of the piezoelectric resonator 220 and the air-tight case 202 into conformity. Therefore, the stress applied to the piezoelectric resonator changes with the change in temperature, and the stress change due to the temperature change appears as an error of the pressure measurement value. In JP-A-2007-57395, the bellows are used for making the pressure measurement value little susceptible to the linear expansion coefficient. However, the effect of the linear expansion coefficient cannot be made zero by the bellows.